1. Field of the Invention
This invention relates to a mullite-alumina composite sintered body having high mechanical strength and heat-resisting property, and a process for producing the same. This invention provides a ceramic material suitably usable as a material for various furnaces or a heat-resisting structural material etc. which requires mechanical strength at a high temperature as well as at room temperature.
2. Description of the Prior Art
Mullite is a compound, having a molar ratio of alumina (Al.sub.2 O.sub.3) to silica (SiO.sub.2) of 3:2, has been known as oxide ceramics having high mechanical strength at a high temperature and is attracting attention as a structural material having heat and oxidation resistance.
It is necessary to employ a chemically processed high purity powder as the starting material so as to obtain high performance mullite ceramics. Starting powdery materials for producing such high performance mullite ceramics can be divided into two categories.
The starting powdery materials for mullite of the first category are produced by preparing mullite precursor, mullitizing the mullite precursor by calcination and then grinding the mullitized powder. Such mullite precursor is prepared from high purity alumina, an aluminum salt or boehmite (aluminum hydroxide) and a silicon compound or colloidal silica. In the mullite percursor, hydroxide or oxide of aluminum and hydroxide or oxide of silicon are homogeneously mixed. The above starting powdery materials for mullite have high purity and fine-grained structures.
The starting powdery materials for mullite of the second category are particles formed of a composite or a homogeneous mixture of alumina (or its precursor) and silica (or its precursor). The particles comprise amorphous phase or a mixture of alumina crystal phase and amorphous phase, being substantially free from mullite phase. Such starting powdery materials can also be employed as the precursor of mullitized powder in the first category.
Phase transformations do not occur during sintering in the preparation methods employing the first category materials, while they occur in the preparation methods employing the second category materials. Therefore, there is a possibility of a great difference in sintering behavior and microstructures, depending upon which category of materials is employed.
Among the methods using mullitized powder in the first category, there are some reports by Kanzaki et al. in which mullite powder is prepared by the process comprising spray pyrolysis of the solution of Al(NO.sub.3).sub.3 and Si(OC.sub.2 H.sub.5).sub.4, calcination and thereafter grinding of the mullitized powder (Yogyo-Kyokai-Shi 93 [7]407-408 (1985), and "Mullite" edited by Somiya, published by Uchida Rokakuho pp. 51-61 (1985). The room temperature bending strength of a sintered mullite body produced by pressureless sintering in the above report does not so much vary with the composition of mullite. The value of the bending strength in the case of sintering at the temperature of 1650.degree. C. for 4 hours is from 320 to 380 MPa and the sintered density is only about 95% of the theoretical density, and therefore the enhancement of the density might be necessary to improve the bending strength. In fact, in the case of sintering by a hot-pressing method, the high bending strength of 500 MPa could be achieved at the stoichiometric mullite composition (containing 71.8 wt % Al.sub.2 O.sub.3). But there has been no report concerning the hot-pressing of mullite containing excessive Al.sub.2 O.sub.3, which is relatively difficult to sinter.
Ismail et al. produced a sintered body having the room temperature bending strength of 405 MPa (J. Am. Ceram. Soc., 70 [1] C-7.about.C-8 (1987), Japanese Patent Appln. Laid-Open No. 61/281013). Such a sintered body is produced by calcining the mixture of boehmite and colloidal silica at the temperature of 1400.degree. C., grinding it to obtain stoichiometric mullite powder and then shaping the powder and sintering it at the temperature of 1650.degree. C. to densify to a relative density from 98.9 to 99.5%.
Hiraiwa et al. achieved the densification up to 99% of the theoretical density by preparing a precipitate from an aluminum salt and colloidal silica, heat-treating the precipitate to form mullite powder and sintering this mullite powder at 1550.degree. C. (Japanese Patent Appln. Laid-Open No. 62/17005).
Hirano et al. obtained high strength mullite and mullite-alumina composite sintered bodies by the method, which employs sodium silicate and an aluminum salt as starting materials, prepares mullite powder having the Al.sub.2 O.sub.3 /SiO.sub.2 weight ratio of from 65/35 to 80/20 by heating and grinding, and sinters this mullite powder at 1600.degree. C. for 2 hours (Japanese Patent Appln. Laid-Open No. 62/56356). With respect to the stoichiometric mullite composition of 72/28 by the Al.sub.2 O.sub.3 /SiO.sub.2 weight ratio, the room temperature bending strength of 43 kg/mm.sup.2 (421 MPa) and the sintered density of 99.7% (as to the theoretical density of 3.17 g/cm.sup.3) was achieved. With respect to the mullite-alumina composite composition of 78/22 by the Al.sub.2 O.sub.3 /SiO.sub.2 weight ratio, the room temperature bending strength of 45 kg/mm.sup.2 (441 MPa) was achieved, but the sintered density was still as low as 96% (as to the theoretical density of 3.27 g/mm.sup. 3).
If the high density can be attained in the composition range containing excessive alumina relative to the stoichiometric mullite composition, the higher strength can be expected. But the high density was difficult to achieve by the above method employing the mullitized powder as a starting material.
Concerning the methods employing powders of the second category which hardly contain mullite phase, first Gardner et al. reported that a density higher than 99% of the theoretical density could be achieved by the method of shaping a compound of 3Al.sub.2 O.sub.3 +2SiO.sub.2 obtained by flame pyrolysis of a solution of 6AlCl.sub.3 +2SiCl.sub.4 and sintering it at a temperature from 1500.degree. C. to 1600.degree. C. (U.S. Pat. No. 3,857,923, Japanese Patent Appln. Laid-Open No. 49/52200). But the mechanical properties have not been reported, and the variation by the composition has not been referred to, either.
Gani et al. prepared Al.sub.2 O.sub.3 -SiO.sub.2 composite powder of various compositions by oxidation of gaseous mixtures of Al.sub.2 Br.sub.6 and SiCl.sub.4 in r.f. plasma, and discussed the properties of the composite powder, especially crystalline phases (J. Mater. Sci., 12 (1977) 999-1009). But they have not referred to the sinterability of the powder nor the properties of mullite obtained by sintering the powder.
Prochazka et al. have shown that optically translucent mullite could be produced by a process comprising the steps of calcining an amorphous oxide mixture, which consisted of from 74 wt % to 76.5 wt % Al.sub.2 O.sub.3 balance SiO.sub.2 with a surface area ranging from 100 to 400 m.sup.2 /g, at a temperature from 490.degree. to 1000.degree. C., preferably at a temperature from 500.degree. to 700.degree. C., thereafter subjecting a calcined compact to the primary sintering at a temperature from 1500.degree. C. to 1675.degree. C. in oxygen or in vacuum, and further sintering it at a temperature from 1700.degree. C. to 1850.degree. C. (U.S. Pat. No. 4,418,024, 4,418,025). With respect to the sintered compact produced by the primary sintering, it is described that said sintered compact was gas-impermeable, and there is no description concerning density, microstructure nor strength.
Debely et al. have shown that the densification al a temperature lower than in the previous methods could be attained by the method comprising the steps of preparing separately a slurry of commercial alumina having an average diameter of 0.29 .mu.m and a slurry of colloidal silica having an average diameter of 0.02 .mu.m, introducing the alumina slurry little by little into the silica slurry to cover the alumina particles with silica, taking out the alumina particles covered with silica by a centrifuge, pressing and sintering (J. Am. Ceram. Soc., 68 [3] C-76.about.C-78 (1985)). They have shown that the densification to 98% of the theoretical density could be attained by sintering at 1400.degree. C. for 1 hour. The composite powder having the composition of 80 wt % Al.sub.2 O.sub.3 and 20 wt % SiO.sub.2 reached full (100%) density by sintering at 1550.degree. C. for 1 hour, and the sintered body comprising mullite and alumina having the sintered grain size of about 0.5 .mu.m was obtained. Debely et al. have not reported the mechanical properties of the sintered body. In this report, it was described that the chemical reaction for mullite formation began at 1490.degree. C. Therefore, it is assumed that the strength of the sintered body is not so high, since unreacted glass phase of silica remains along grain boudaries, while the sintering proceeds at a low temperature because of the existence of glass phase of silica.
Kawado obtained a sintered body densified to 99% of the theoretical density by the method which comprised employing alumina produced by a vapor-phase reaction and alkoxide of silica as starting materials, shaping after gelation, and sintering the shaped body. But the room temperature strength was still as low as 280 MPa (Japanese Patent Appln. Laid-Open No. 62/46955, No. 62/46956).
The above prior arts may be summarized as follows:
In the case of producing sintered bodies from mullitized powder, the densification is difficult because of the poor sinterability of mullite. Especially, at a composition containing excessive alumina relative to the stoichiometric mullite composition, the densification is more difficult since remaining glass phase of silica is not present. From the report by Hirano et al. (Japanese Patent Appln. Laid-Open No. 62/56356), if the high density is attained without causing much grain-growth, high strength is expected with respect to the mullite-alumina composite sintered body having the composition of about 78 wt % Al.sub.2 O.sub.3 and about 22 wt % SiO.sub.2. But it has not been possible by the prior arts.
On the other hand, in the case of producing sintered bodies from the powder which hardly contains mullite, it is possible that the sintering proceeds at a low temperature, mainly because of the existence of glass phase of silica. But the chemical reaction to form mullite from this glass phase of silica and alumina is difficult to occur. Especially in the case large alumina particles are used, the strength is not high, because the reaction is never completed and a large amount of the glass phase remains.